Fig. 1

Details of the phylogenetic relationships and the morphology of the species in this study. Phylogeny of the four species (left) next to line drawings of the male copulatory organs (stylets) and sperm, and light microscopic images of lightly squeezed live worms. The type of mating (reciprocal/hypodermic) is indicated above the species name. The phylogeny (see also Results) is rooted at the branch leading to M. pusillum since this represents the deepest split in the genus (see [16]). The grouping of M. lignano with M. hystrix has maximal support (in both the ultrafast bootstrap as well as the Shimodaira–Hasegawa–like approximate likelihood ratio test), which suggests independent origins of the hypodermic mating syndrome in M. hystrix and M. pusillum. The scale bar represents substitutions per site, and the numbers next to the nodes give the number of gene duplications that occurred according to the OrthoFinder analysis (see also Methods; the amino acid alignment, the inferred phylogeny, and the log file of the IQ-TREE analysis are provided in Additional file 1: “Amino acid alignment of one-to-one orthologs”; Additional file 2: “Maximum likelihood phylogeny” and Additional file 3: “IQ-TREE logfile”). The stippled lines on the light microscopic images show the intended cutting level for the regenerant treatment (see also Methods)